1. Field of the Invention
The present invention relates to the preparation of novel metal organic precursor compounds comprising at least one metal from Group IIb and at least one element from Group VIa of the Periodic Table. More specifically, the II/VI compounds are useful as single source metalloorganic tellurium or selenium, or sulfur, zinc, cadmium or mercury containing precursors useful to prepare semiconducting materials, e.g. as thin films, having precisely defined metal ratios. ##SPC1##
2. Description of Related Art
Semiconducting thin-film alloys can be comprised of various combinations of elements from Group IIb (II) and VIa (VI) of the standard Periodic Table. These alloys are increasingly important due to their useful electronic, optoelectronic, magnetooptic and piezoelectronic properties. These physical, chemical and electrical properties make the II/VI alloys of interest for a range of applications including, but not limited to, photovoltaic cells, infrared windows, light emitting diodes, blue-green lasers, and the like.
In the conventional processing, the growth of II/VI thin films usually involves the use of two or more different volatile, toxic metal alkyl precursor compounds, i.e., one metalloorganic compound for each metallic element to be deposited. Because each metalloorganic precursor has different reactivities, undesirable pre-reactions often occur prior to mixing and side reactions during processing usually occur which seriously limit the quality and the usefulness of the thin films of metal alloy which are produced.
These undesirable reactions can lead to difficulties in controlling the Group II/Group VI metal ratio in the resultant films, because one of the metal alkyl precursors is depleted before deposition of the desired metal alloy film on the substrate can occur. The toxicity of currently used II/VI precursor metallo-organic compounds also leads to considerable safety and environmental difficulties in storage, processing and eventual disposal. The use of two or more metalloorganic compounds requires the extra capital expense of a second (or third) precusor compound line to the reactor in which the metal alloy is produced.
At the present time, the only general synthesis route to prepare atypical metal tellurolates (or similar selenium, germanium or tin containing compounds) having a low molecular weight and solubility in organic solvents involves a metathesis reaction between an alkali metal tellurolate and a metal halide. See P. J. Bonasia et al., Journal of the Chemical Society, Chem. Comm., 1990, p. 1299. S. A. Gardner, Journal of Organometallic Chemistry, 1980, Vol. 190, p. 280, and I. Davies et al., Inorganic Nuclear Chemistry Letters, 1976, Vol. 12, p. 763.
The tellurolysis pathway outlined below has not been previously reported: EQU L.sub.n M-X+H-TeR.sup.3 .fwdarw.L.sub.n M-TeR.sup.3 +H-X
L is a ligand PA1 R.sup.3 =alkyl, aryl PA1 X=alkyl, amide, alkoxide, etc. PA1 n is 0-5, PA1 (L, R.sup.3, X, n defined here for this paragraph only.) PA1 (i) the choice of metal halide starting materials, PA1 (ii) tellurolate anions are quite reducing, and PA1 (iii) the presence of strong donor molecules (either as solvent or ligated to the tellurolate salts) often interferes with product purification. PA1 wherein M is selected from the Group IIb elements of zinc, cadmium, or mercury; PA1 A is selected from amide, alkyl having from 1 to 10 carbon atoms, aryl, substitued aryl, or --Q'--Y'--(Si--R'.sub.3).sub.3 L.sub.2 wherein L is selected from nothing or a Lewis base ligand, PA1 Q and Q' are each independently selected from Group VIa elements of sulfur, selenium, or tellurium; and PA1 Y and Y' are each independently selected from carbon, silicon, germanium or tin, and PA1 R and R' are each independently selected from alkyl having from 1 to 10 carbon atoms, aryl or substituted aryl. PA1 Prefereably, the metal compound has A as --Q'--Y'--(Si--R'.sub.3).sub.3 and L is nothing, especially wherein R.dbd.R', Q.dbd.Q', and Y.dbd.Y. PA1 wherein M is selected from the Group II b elements of zinc, cadmium, or mercury; PA1 A is selected from amide, alkyl having from 1-10 carbon atoms, aryl or substituted aryl, or --Q'--Y'--(Si--R'.sub.3).sub.3 L.sub.2 wherein L is nothing or a Lewis base ligand, PA1 Q and Q' are each independently selected from Group VIa elements of sulfur, selenium, or tellurium; and PA1 Y and Y' are each independently selected from carbon, silicon, germanium or tin, and PA1 (A) contacting L'.sub.3 --Z--Y--(SiR.sub.3).sub.3 wherein L' is independently selected from ligand L groups; PA1 Z is independently selected from lithium, sodium, potassium, calcium, barium or strontium; and PA1 Y and R are defined hereinabove, with a metal powder Q wherein Q is defined hereinabove, in a hydrocarbon solvent under an inert anhydrous atmosphere for between about 0.0i and 2 hr at between about -20.degree. and +3.degree. C., followed by removal of the hydrocarbon solvent; PA1 (B) contacting the product of step (A) with a strong acid to produce and separate EQU H--Q--Y--(SiR.sub.3).sub.3 PA1 Q, Y and R are defined hereinabove; PA1 (C) contacting the product of step (B) with either: PA1 (D) recovering the metalloorganic compound of structure (I), where L is nothing; and PA1 (E) optionally contacting the product of step (D) with ligand L to produce the organometallic compound where L is a Lewis base. PA1 (a) subjecting the metalloorganic compound (I) described above to a temperature of between about 150.degree. and 500.degree. C. in an anhydrous vacuum between about ambient pressure to 10.sup.-6 Torr (e.g. 10.sup.-2 tp 10.sup.-6 Torr), especially the metal alloy producing process wherein A is --Q'--Y'--(SiR'.sub.3).sub.3 and R.dbd.R', Q.dbd.Q', and Y.dbd.Y'.
Analogous reactivity is documented for alcohols, thiols and selenols, where reactions are extremely flexible and with respect to the choice of R and the leaving group X. In addition, these reactions are best carried out in non-polar solvents. The use of silicon, germanium, or tin-containing precursors with sulfur, selenium or tellurium is not reported. Further, for tellurolysis, a major drawback has been that known tellurols are thermally unstable that are difficult to isolate and purify.
Some general reports of the production of II/VI materials using one, two or more separate metalloorganic compounds include, for example:
M. B. Hursthouse et al. (1991) Organometallics, Vol. 10, pp. 730-732, describes compounds of mixed alkyl dialkylthiocarbamates of zinc and cadmium. These precursors are then used for deposition of semiconductors by metallo-organic chemical vapor deposition (MOCVD).
Y. Takahaski et al. (1980) Journal of Crystal Growth, Vol. 50, p. 491, describes the preparation of precursors of cadmium or zinc dimethylthiophosphinates for the production of cadmium sulfide or zinc sulfide.
J. O. Williams et al. (1982) Thin Solid Films. Vol. 87, L1, describes the growth of highly ordered sulfide films using metal dimethylthiophosphinates.
A. Saunders et al. (1986) in "Ternary Multiary Compound" in Proceedings of the International Conference 7th, published 1987, (see Chemical Abstracts, 1988, Vol. 108, #66226h) describes the growth of sulfide films from zinc and cadmium thiocarbamates).
D. M. Frigo et al., (1989) Journal of Crystal Growth, Vol. 96, p. 989, describes sulfides of excellent crystallinity grown using bis(diethyldithiocarbamates).
M. Bochmann et al., (1989) Angew. Chem. Int. Ed. in English. Vol. 111, p. 414, discloses the preparation of low-coordination number complexes of cadmium and zinc with sterically hindered thiols and selenols, such as 2, 4, 6-tri-tertiary-butylbenzene-thiol.
G. N. Pain et al. in Polyhedron, (1990) Vol. 9, #7, pp. 921-929, discloses the preparation of organometallic cadmium, mercury and tellurium compounds which are used as precursors to metal alloys. Pain et al. does not teach any silicon containing precursors.
J. G. Brennan et al. in Chemical Materials, (1990) Vol. 2, pp. 403-409 discloses the use of separate metalloorganic II/VI precursors useful in the preparation of metallic thin films.
S. M. Stuczynski et al. in Inorganic Chemistry (1989), Vol. 28, #25, p. 4431 and 4432 disclose the formation of metal-chalcogen bonds by the reaction of metal alkyls with silyl chalcogenides. However, they do not teach or suggest the preparation of all metals in a single precursor compound.
D. W. Kisker in Journal of Crystal Growth (1989), Vol. 98, p. 127-139, discusses the II-VI family of semiconductor alloys as obtained by organometallic vapor phase epitaxy (OMVPE), particularly in applied optoelectronics.
All of the references, patents, standards, etc. referenced in this application are incorporated herein by reference.
The problems in this art remain, i.e. the reactive precursors, undesirable pre-reactions and/or side reactions. Additional disadvantages include but are not limited to:
None of the above cited references describe a single stable silicon-containing metalloorganic compound having the metal atoms present in a particular ratio, which ratio can be varied by judicious choice of metalloorganic substitutents.
It would be extremely useful to have the metals of interest in a single stable metalloorganic precursor compound so that the ratio of the metals deposited as an alloy upon decomposition can be more precisely controlled. The present invention provides such precursor compounds and processes to produce them.